Articles & Tutorials

Cisco Certification: An Introduction To Multilayer Switching And SVIs
By Chris Bryant

Sure, you have to know all about SVIs and multilayer switching to earn your CCNP, but you also need to know about them because they’re so commonplace in today’s networks!

Why? We can use an SVI to allow inter-VLAN communication on a multilayer switch, eliminating the need for a separate router to get involved.

In this example, we have Host_1 in VLAN 11 and Host_3 in VLAN 33. Before we begin configuring, we'll send pings between the two hosts. (We’ll use Cisco routers for the hosts in this lab, which is why the ping output may look familiar!)

HOST_1#ping 30.1.1.1

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 30.1.1.1, timeout is 2 seconds:.....

Success rate is 0 percent (0/5)

HOST_3#ping 20.1.1.1

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 20.1.1.1, timeout is 2 seconds:.....

Success rate is 0 percent (0/5)

As we expect, there’s no IP connectivity between the hosts. Assuming the appropriate switch ports are already placed in VLAN 11 and VLAN 33, we’ll now create two switched virtual interfaces on this multilayer switch. One will represent VLAN 33, and the other will represent VLAN 11.

SW1(config)#int vlan11

01:30:04: %LINK-3-UPDOWN: Interface Vlan11, changed state to up

01:30:05: %LINEPROTO-5-UPDOWN: Line protocol on Interface Vlan11, changed state to up

SW1(config-if)#ip address 20.1.1.11 255.255.255.0

SW1(config-if)#int vlan33

01:30:11: %LINK-3-UPDOWN: Interface Vlan33, changed state to up

01:30:12: %LINEPROTO-5-UPDOWN: Line protocol on Interface Vlan33, changed state to up

SW1(config-if)#ip address 30.1.1.11 255.255.255.0

At this point, we’ve still got a problem when we look at the routing table.

SW1# show ip route

Default gateway is not set

Host Gateway Last Use Total Uses Interface

ICMP redirect cache is empty

The problem is that we don’t have a routing table, because IP routing is off by default on a multilayer switch. Let’s turn IP routing on and then check the routing table.

SW1(config)#ip routing

SW1(config)#^Z

SW1#show ip route

Gateway of last resort is not set

20.0.0.0/24 is subnetted, 1 subnets

C 20.1.1.0 is directly connected, Vlan11

30.0.0.0/24 is subnetted, 1 subnets

C 30.1.1.0 is directly connected, Vlan33

Now we’re in good shape! The hosts need to have their default gateway set to the appropriate SVI IP address; here, Host_1 should have its gateway set to 20.1.1.11, and Host_3 to 30.1.1.11. After doing so, inter-VLAN communication is now in place, as shown by the ping output below.

HOST_1#ping 30.1.1.1

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 30.1.1.1, timeout is 2 seconds: !!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 1/1/4 ms

HOST_3#ping 20.1.1.1

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 20.1.1.1, timeout is 2 seconds: !!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 1/2/4 ms

In a future Cisco certification tutorial, we’ll take a look at configuring a multilayer switch port as a routed port, and assign an IP address to that port. See you then!